Platelet-derived growth factor receptors (PDGFRs), alpha and beta, are receptor tyrosine kinases which mediate mitogenesis, cell survival, and chemotaxis. PDGFR signaling has been implicated in neoplasia and embryogenesis. Both receptors signal via similar, albeit subtly different, biochemical mechanisms in vitro. Targeted deletions of the two PDGFRs produce unique phenotypes, suggesting that each mediates specific biological functions. The goal of this project is to understand how the biochemical signaling events generated by the PDGFRs function to regulate mammalian embryogenesis. The specific objectives are outlined below. 1. Determine whether the specific physiological functions of the two PDGFRs are due to distinct biochemical mechanisms of signal relay. PDGFR "knock-in" mice will be created in which the genomic locus of one receptor isoform, will be disrupted and replaced by a gene encoding the catalytic domain of the other receptor isoform by homologous recombination in ES cells. Phenotypic analysis of the resulting mice should provide an indication as to the degree of functional over lap of the signals transmitted by the two PDGFRs in vivo. 2. Identify the mechanistic basis of the PDGFalphaR-deficient mutant phenotype. Initiation of PDGFR signaling involves activation of a number of SH2 domain-containing signal relay proteins. To elucidate the contribution of specific PDGFalphaR-activated signals to embryogenesis, the knock-in approach will be utilized to replace the endogenous alpha- receptor with a series of mutant receptors that selectively activate only one of these proteins. The knock-in approach provides a unique opportunity to explore the physiological importance of distinct PDGFR-mediated signaling events in a n intact mammal and may provide insight as to the role(s) of PDGFRs in disease.